Abd: we don't. Cold fusion is a mystery, and the mystery exists based on experimental evidence. No cold fusion theory is necessary to recognize the experimental evidence, and physicists, in general, will not look carefully at cold fusion theories until they know the experimental evidence that establishes the mystery.

The largest practical problem in cold fusion is the unreliability of the reaction. That problem did not prevent the demonstration of cold fusion reality through the correlation of heat with helium[1], because reliability was sufficient for that.

The principal difficulty in measuring the heat/helium ratio is capturing all the helium. Leakage is a problem, but one that has been adequately addressed. (Most importantly, leakage could not create a heat/helium correlation; the very unreliability of the reaction causes apparently identical experimental circumstances to produce differing amounts of heat, from none at all up to heat beyond the possibility of chemistry, hence additional controls are not necessary. A heat/helium experiment is self-controlling. (If it were reliable, one would need, then, to vary the heat production through controls; that has been done with hydrogen controls, but the point is that it isn't necessary, not that it isn't useful.)

No cold fusion theory advanced so far has been able to predict reaction rates and enhance reliability.

The most difficult aspect of cold fusion theory is how the energy released is distributed without gamma rays or very hot particles, and it is not necessary to know how this happens to engineer more reliable cells. Obviously, a comprehensive theory would be useful for engineering, but, so far, nothing published has come close to this.

Cold fusion is a mystery, something that can be shown to happen, experimentally, but for which no explanation has proven adequate.

Because these other reported effects may be due to a different mechanism than what is behind the FP Heat Effect, it is not essential that a successful theory explain them. Historically, though, many cold fusion theories were first proposed before it was clear that heat and helium were the only major product of the reaction, and Storms complains that some theories still focus on possible new physics, or unexpected physics, without respecting the known chemistry and experimental conditions under which cold fusion is observed.

There have been many proposed mechanisms to explain cold fusion, including many which are no longer considered plausible, but which may be of historical interest. Storms notes that the problem is not that there is no theory, but that there are too many. None are considered by him to adequately explain all the observed phenomena. For an early review of cold fusion theory, which is still of great value, see:

We briefly summarize the reported anomalous effects in deuterated metals at ambient temperature, commonly known as "Cold Fusion" (CF), with an emphasis on important experiments as well as the theoretical basis for the opposition to interpreting them as cold fusion. Then we critically examine more than 25 theoretical models for CF, including unusual nuclear and exotic chemical hypotheses. We conclude that they do not explain the data.

In this section we will list and briefly summarize each proposed theory. On a page dedicated to each theory, we will example the evidence and arguments for and against each theory, as well as some history of the theory.

Most mechanisms [proposed] either involve the deuteron being pushed over the Coulomb barrier by localized energy or electrons shielding the charge on the deuteron in various ways. Variations on these basic mechanisms have generated over 300 published papers. A few models even propose neutrons to be involved or that the Coulomb barrier simply disappears under certain conditions. With so little information available initially, theory was based mostly on imagination and speculation, including the approach that encouraged Fleischmann and Pons.[1-3] While much more information is now available to define the limits of theory, this situation has not changed significantly [this was published in 2007].

1. 1990 Fleischmann, M. An Overview of Cold Fuson Phenomena. in The First Annual Conference on Cold Fusion. 1990. University of Utah Research Park, Salt Lake City, Utah: National Cold Fusion Institute.

Storms conclusion: "A variety of plausible and implausible NAE models have been explored, with little evidence that an actual one has been identified."

Storms also notes the "multiple reaction" possibility:

A distinction needs to be made between reactions resulting in measurable heat and those producing very little nuclear product, such as transmutation, tritium, and neutron production. The difference in rates is so huge that a variety of NAE may be operating by different mechanisms.

This is a general category, under it we will cover Takahashi's Tetrahedral Symmetric Condensate theory and Kim's Bose-Einstein Condensate theory. We may also classify with this the work going on which is looking at nanoparticle palladium as if it, and the absorbed deuterium, behaves as a single quantum entity, perhaps similarly to a Bose-Einstein Condensate.

Takahashi, a physicist, first found, through the bombardment of palladium deuteride targets by accelerated deuterons (at below ordinary fusion energy), evidence that multibody fusion was enhanced by a factor of 10^26. He then began studying possible multibody fusion mechanisms using quantum field theory. In particular, he has published extensive analysis of what would occur, as predicted by the theory (this is apparently standard quantum theory) should a particular physical configuration of deuterium atoms form.

This would be four deuterons in a "tetrahedral symmetric" configuration. It appears that the electrons are included in the calculations, either three or four of them. He does not propose a mechanism for this TS configuration would conform (it would take some energy, but how much has not been calculated, so it is not known if that energy is within range of the kinetic energies available at room temperature.) If the configuration forms, however, which presumably means that the four deuterons are in position and with low relative velocity, a Bose-Einstein Condensate is predicted to form and collapse and fuse to beryllium-8 within a femtosecond. This Be-8 would then decay into two helium nuclei. No gamma ray.

However, the exact decay mechanism is not predicted yet. The behavior of a fused nucleus within a BEC is not known. The energy might be distributed among all the particles, which would be, in this case, one Be-8 nucleus and four electrons.

There is other evidence which indicates that larger clusters may fuse, and Takahashi has not proposed the TSC as the only mechanism, it is remarkable simply because, for the first time, a quantum field theory calculation predicts fusion under certain not-so-impossible physical conditions. There is much skepticism among cold fusion researchers about this theory; most notably Storms has criticized it because the energy to form the TSC is, he believes, missing.

(A concept of this is to imagine two deuterium molecules. If they approach each other in free space, with sufficient velocity that they would be headed for TSC, the repulsive forces present would cause the deuterium molecules to dissociate. For the deuterium to not dissociate, something would have to resist that dissociation. That would be, presumably, the lattice. One experimental fact is that the observed reaction rate generally increases with temperature. That could be explained by the increased availability of deuterium molecules with the needed initial energy.)

Takahashi has proposed that the excited Be-8 nucleus, before decaying, would emit photons in a series of transitions, and, if it has not fissioned yet, it could reach the ground state. From there, when it decays, each alpha particle (helium nucleus) would have about 90 KeV of energy, which is well above the 20 KeV limit established by Hagelstein based on the lack of signs of particles with higher energy. (These photons would be relatively low energy, they would not be "gamma rays," and it is plausible that most of the 47.6 MeV energy ultimately released by the fusion of four deuterons to an ultimate helium product could be transferred to the environment by these photons.)

If, however, larger clusters form and only one or a few fusions occur within the cluster, the energy could be distributed among larger numbers of component particles.

Model proposed by Yeong E. Kim. Postulates that a Bose-Einstein Condensate (BEC) state can form in both light water and heavy water systems on micro/nano-scale metal grains or particles. Metallic hydride grains and defects act as barriers that slow down mobile protons/deuterons. When the average velocity and kinetic energy of flowing hydrogen/deuterium becomes low enough localized regions within the hydride lattice/matrix transform into atomic BEC sites. As a result the protons/deuterons DeBroglie Wavelengths become sufficiently large and overlap. Fusion rates and probability are not dictated by the Gamow factor at these sites. Instead they depend on the probability of ground-state occupations of overlapping Bosons. The Coulomb Barrier is therefore bypassed. Also, radiation is distributed and dissipated throughout the BEC clusters, thereby eliminating harmful gamma emissions.

The Ronald Brightsen Nucleon Cluster Model explains the excess energy and byproducts observed in LENR reactions as resulting from overlap of quantum wavefunctions (e.g., "cluster fusion") of mass asymmetric nucleon clusters (2 vs 3 amu). The "cluster fusion" is between one matter cluster having positive effective mass and positive gravity potential, interacting with a second antimatter cluster with negative effective mass and negative gravity potential. The model predicts that reported LENR reactions, such as introduction of hydrogen isotopes into palladium lattice, or reported LENR reactions between isotopes of nickel and atomic hydrogen gas, are neither a weak force interaction nor a classical fusion of nuclei, but can be explained as an annihilation of clustered bags of matter and antimatter quarks, resulting in transmutation of isotopes and excess energy in the form of pions and their decay products. The Brightsen model adopts an understanding of nucleon shell structure beyond the current Standard Model that requires the presence of resonating group structures (e.g., nucleon clusters, both matter and antimatter) within stable isotopes, with energy of the system and reaction cross sections derived from application of Fredholm determinat, a theory first adopted in 1937 by John Wheeler, today largely ignored (Phy. Rev. 52:1107).

See an examination of the Brightsen Nucleon Custer Model by Robert Bass at [5]

(This section is included here for historical interest. Sometimes newcomers to the field suggest that stray muons might be involved, but there are many reasons why this idea isn't long maintained by anyone.)

Muon-catalyzed fusion itself was proposed as a theory to explain cold fusion at one point, from cosmic-ray muons setting up occasional reactions, which would be erratic, perhaps the lattice somehow causes the muon preservation to be much higher than in ordinary MCF in pure hydrogen or deuterium, so that a single muon would cause many more reactions than in the known form.

This theory fails because the muon will quickly be captured by the highly charged Pd nucleus, at which point it can no longer catalyze fusion events. Even in pure hydrogen isotopes, muons do not live long enough to generate more than a few hundred fusions, and the muons must be replenished, since they are unstable.

Further, muon-catalyzed fusion shows the same branching ratio as ordinary hot fusion. So there would be copious neutrons and tritium, if muon-catalyzed fusion were responsible for more than a tiny part of the Fleischmann-Pons Heat Effect.

Model proposed by KP Sinha and Andrew Meulenberg. Postulates that fusion can occur between hydrogen or deuterium within solid state lattice systems that contain linear defects. Linear defects are degraded surface phenomena caused by heavy loading processes where mobile protons/deuterons are then able to congregate. These interstitial surface anomalies are sometimes referred to as a sub-lattice in ELT and have recently been correlated with Storms' Nano-crack NAE by Meulenberg himself at ICCF-18.[2]

Absence of radiation is accounted for by two mechanisms. Firstly, gradual dissipation and exchange of thermal energy occurs between ion chains and the lattice before H+H-/D+D- fusion takes place. Secondly, H+H-/D+D- that are deeply bound with lochons do not occupy an excited state above the fragmentation level which prevents gamma transitions after fusion.[3][4]

Model proposed by Edmund Storms. Envisions Nano-crack Nuclear Active Environments (NAE) forming in condensed matter systems as a byproduct of stress relief due to electrolysis or gas loading. The formation of the NAE occurs during the activation phase following loading and before significant reactions initiate. Flux through the system brings hydrogen or deuterium into and around the Nano-Crack NAE. The more NAE present and higher rate of flux, the more likely ions will settle into the NAE.

Ions within the NAE naturally seek to settle themselves into a low energy state. They arrange themselves into linear, covalent bonded Hydroton (Metallic Hydrogen) chains/stacks constituting an alternating arrangement of protons and electrons. Associated Gibbs energy is also released during this settling process thereby stabilizing the NAE. When hit with enough systemic vibration these Hydroton stacks/chains begin to resonate with one another. As the ions oscillate back and forth they continuously emit low energy photons until they lose enough mass-energy to allow for the electron shield to initiate fusion between nucleons. The electron that helped reduce the Coulomb barrier is sucked into the final product nucleus. High frequency gamma radiation is not detected because there is no longer enough energy left within the nuclei (as a result of numerous low energy photon emissions) at the moment of fusion. Material continually flows into and out of the NAE and the process perpetuates itself.[5][6][7]

Model proposed by Peter Hagelstein. Proposes that D-D fusion can occur under special conditions within vacancies throughout the surface of a chemical lattice in PdD systems. Molecular D2 and nuclear lattice-based He4 form two-level systems that contain large transition energy. They become coupled with other D2-He4 systems via resonant phonon excitations (aka low-energy harmonic oscillators) throughout the lattice (aka Spin-Boson Model). Phonon modes are initiated by flux through the near-surface of the cathode. Increased phonon excitation and energy transfer rate is achieved by reduction of interference augmented through a "loss process". This rapid, distributed, small energy quanta exchange is what allows for D-D fusion to occur at lattice sites without correlated radiative effects.[8][9][10]

Model proposed by John Hadjichristos of Defkalion Green Technologies (DGT). When plasma glow discharges are applied to a modified NiH system molecular H2 is thought to break up and form into dipolar Rydberg State Hydrogen (RSH). Similar to Storms, DGT assumes that the formation of a novel, Nano-NAE is a necessary condition to initiate reactions. DGT's NAE is an engineered, evolving vacancy site found within a mixture of Ni crystal micron powder and Ni foam. The NAE changes shape and atomic separation in response to applied temperatures. RSH is guided toward these NAE by magnetic fields where they form into clusters. Once in this condensed state each RSH proton is very close to its electron and is capable of behaving as a disguised neutron for a brief moment. During these short intervals Coulomb forces between nuclei approach zero.

DGT refers to this as a HENI process. This can mean either Heat Energy from Nuclear Interactions -or- Heat Energy from Nanoplasmonic Interactions. Nanoplasmons are quantized quasi-particles formed out of collective plasma oscillations in a free electron gas. At this moment DGT considers HENI a hybridized reaction process that produces heat from a mixture of both weak interactions and nucleon-synthesis. Similar to W-L theory, HENI theory postulates that heavy electrons provide gamma shielding.[11][12]